The present invention relates to the encapsulation of microelectronics chips.
Background: General Encapsulation
After the fabrication of semiconductor wafers, there still remain the processes of protecting the sensitive wafers from environmental hazards, as well as providing connections to other devices. One of the most common solutions to these needs involves first attaching individual dies to a leadframe, then enclosing the die and portions of the leads in a covering of plastic.
FIG. 2 shows a typical leadframe cluster 21, in this case, for a 40-pin dual in-line plastic package. This leadframe cluster contains six separate leadframes 23, although this number can be modified, depending on the final size of the package. Each leadframe 23 contains the contacts for one semiconductor circuit. In the middle of the leadframe 23, there is an area where the die will be attached—the die paddle 20. Individual leads 22 fan out from the die paddle, on two or four sides of the die paddle, depending on the type of package. Shorting bars 24 are located between the leads for different packages and large shorting bars 25 connect together multiple ones of the frames. These shorting bars provide stability to the framework-during assembly and packaging, but will not be part of the final packages. Additionally, dam bars 26 are located just outside what will be the final package size and are used to prevent excess seepage of molten plastic out of the package during encapsulation. In at least some applications, especially where a large number of leads are closely packed, a small cutout on the inside edge of the dam bars, commonly called a gate window 27, is used to provide an opening for the encapsulant to be routed into the area surrounding the leads.
After individual semiconductor dies are separated from the wafer, they are attached to the die paddle 20 of a leadframe 28, using one of several available materials for that purpose. Thin wires are then bonded to each of the contacts on the chip, with their other end being bonded to one of the leads 22 on the leadframe. In this manner, electrical connections to the chip will be carried outside the finished package. After bonding, the leadframes will be encapsulated, with the most common method being by transfer molding in a cavity-chase mold.
Background: Chase Cavity Molds
FIG. 3A shows the overall layout of a chase cavity mold 31 which will be used to encapsulate the leadframe cluster shown in FIG. 2. The mold is of a durable metal construction, with interchangeable parts. A central pot 30 holds the encapsulant material while it is being melted, while runners 32 route the melted plastic from the pot 30 toward the enclosed leadframes 23. The portion of the mold which contains the leadframes and the runners in their immediate vicinity is an insertable piece which can be changed to allow for different packages to be encapsulated using the same chase mold.
Dotted lines 21 show how one leadframe cluster sits in the mold, with individual cavities 34 surrounding the individual leadframes in the shape of the desired package. Small gate runners lead to gates 36, which open into the individual cavities 34 to allow the plastic to enter. Because of the hardeners used in the encapsulant, the gate, where the flow is rapidly constricted, wears more heavily than other parts of the mold. For this reason, the gates are constructed on pins, having a circular or ovoid cross-section, which can be inserted or removed from the mold when necessary.
Sometimes the cavities are in two rows on either side of the runners, as shown in FIG. 3A. In this case, a primary gate 36 leads the plastic into a first cavity, while a secondary gate 38, having a different shape from the primary gates, routes the molten plastic from the first cavity into a second package.
The mold layouts of FIGS. 3A and 3B are considered unbalanced, because the runners to different cavities have different lengths. This contrasts with mold layouts in which the runners to all cavities are the same, as shown in FIG. 9, considered a balanced mold.
FIG. 10 shows a cross-section of a prior art mold, showing one cavity and its associated runner. The top half 1010 of the mold contains the top half of the cavity 34, plus relief for the presence of the leadframe 23, while the bottom half 1020 of the mold contains the bottom half of the cavity 34, plus the runner 32 and gate 36. The runner and gate will become closed channels when the mold closes, with the additional side formed by the opposite half of the mold and/or the leadframe. This drawing also shows the presence of the chip 1030 to be encapsulated, its bonding wires 1032, and the ejector plate 1040 and ejector pins 1042 which will be used to remove the encapsulated leadframes from the mold.
After the encapsulant has been distributed and cooled, the mold halves are separated and the ejector pins are used to remove the encapsulated leadframe cluster from the mold.
FIG. 7 shows a portion of the encapsulated leadframe cluster after it is removed from the mold. The section on the left shows how flash 710 can form on the leads during encapsulation as plastic leaks between the two mold halves. A “dejunking” operation is used to remove this flash, using, e.g., mechanical abrasion, to give the results on the right. After dejunking, the individual components are punched out of the leadframe. The dam bars and shorting bars are removed using an automatic punch press and the leads are trimmed, then formed to the desired configuration by bending them using a thin anvil.
The final package is shown in FIG. 6. Here again is seen the die 1030 attached to the die paddle 20, connected by bond wires 1032 to the leads 22, and surrounded by encapsulant 62.
Background: Gate Designs
FIG. 5 shows a side view of a mold, showing the relationship of the primary gate pin 52 and the secondary gate pin 54 to the upper 1010 and lower 1012 portions of the mold and to the runner 32. FIGS. 5A–C show three views of a gate pin. As seen in FIG. 5A, the gate pin 50 has a circular shape which can be slid into the mold. This will be connected to a portion of the pin which does not have a symmetric shape, to allow it to be inserted in only one position. A groove 52 runs across the pin, forming a channel which will carry molten encapsulant into the package. An inlet portion 54 of the pin has a depth which corresponds to the depth of the runner; an outlet portion 56 has a depth which corresponds to the package. The depth of the groove 52 varies from that of the inlet portion 54 to thai of the outlet portion 56. FIG. 5B shows a side view of the pin as it would be seen from the package. Outlet portion 56 can be seen, with portions of the channel which are hidden shown as dotted lines. Finally, FIG. 5C shows the pin in cross-section along z–z□ of FIG. 5A. In this diagram, it is easier to see how the depth varies across the pin.
FIGS. 8A–B diagrammatically show cross-sections of two prior art packages within the mold, with only the gate pin inserts shown cross-hatched. Note that in FIG. 8A, the angle between the gate and the leadframe is initially 30 degrees or greater as it leaves the runner, although this angle decreases to about 12 degrees prior to the junction with the package. The gate of FIG. 8B has a smooth arc, so that the angle at which it approaches the leadframe tapers off to a gentle slope near the edge of the package. Both these designs enable the smooth flow of the compound into the package, avoiding pinhole voids near the gate. Note that in both of these examples, the gate runs all the way to the edge of the package area, where it will open either directly into the package or into the gate window in the leadframe. This means that a substantial thickness of excess plastic, i.e., the plastic which cools inside the gate itself, will need to be trimmed away at trim time.
Examples of packages which have been encapsulated using prior art gates are shown in FIGS. 11A–B. In both of these photographs, encapsulation has been done, but the leadframes are still intact and the encapsulant which was present in the gate is still attached to the package. In the photos, you can also see the faint line in the plastic which shows where the gate pin was located. In FIG. 11A, a primary gate is seen leading from the runner, across the dam bar, and all the way up to the package. The edge of the gate window can just be seen near the edges of the gate/package junction. In FIG. 11B, a secondary gate runs over the dam bar between two packages, between their respective gate windows. After this point, the remainder of the gates are removed, along with the excess portions of the framework.
Background: Problems of Thin Packages
One trend in packaging today is that the packages are getting thinner, with thinner layers of plastic overlying the chip. This leads to greater susceptibility to cracking and chipping of the package during necessary processing steps. For example, at trim and form, a pinch cut is used to remove the plastic which was in the gate section of the mold at the time the mold was cooled. This can cause stress on the overall package and lead to cracking.
Virtual Gate Design for Thin Packages
The design disclosed herein includes a gate insert which, prior to or at the edge of the package, has a depth no deeper than the thickness of the leadframe. Here, within the dam/shorting bars of the leadframe, the encapsulant flows into the package using only the vertical space which exists between the leads, thus the term “virtual gate”. Additionally, the gate maintains an angle of approach to the leadframe which is 30 degrees or greater.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:                reduces gate chip;        improves yield;        extends mold tool life (i.e., gate wears at a slower rate);        eliminates pinch cut at trim and form;        reduces external stress on package.        